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FreeBSD-SA-20:28.bhyve_vmcs                                 Security Advisory
                                                          The FreeBSD Project

Topic:          bhyve privilege escalation via VMCS access

Category:       core
Module:         bhyve
Announced:      2020-09-15
Credits:        Patrick Mooney
Affects:        All supported versions of FreeBSD.
Corrected:      2020-09-15 21:28:47 UTC (stable/12, 12.2-STABLE)
                2020-09-15 21:43:41 UTC (releng/12.2, 12.2-BETA1-p1)
                2020-09-15 21:43:41 UTC (releng/12.1, 12.1-RELEASE-p10)
                2020-09-15 21:28:47 UTC (stable/11, 11.4-STABLE)
                2020-09-15 21:43:41 UTC (releng/11.4, 11.4-RELEASE-p4)
                2020-09-15 21:43:41 UTC (releng/11.3, 11.3-RELEASE-p14)
CVE Name:       CVE-2020-24718

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I.   Background

bhyve(8) is a hypervisor that supports running a variety of guest operating
systems in virtual machines on AMD and Intel CPUs.

II.  Problem Description

AMD and Intel CPUs support hardware virtualization using specialized data
structures that control various aspects of guest operation.  These are the
Virtual Machine Control Structure (VMCS) on Intel CPUs, and the Virtual
Machine Control Block (VMCB) on AMD CPUs.  Insufficient access controls allow
root users, including those running in a jail, to change these data
structures.

III. Impact

An attacker with host root access (including to a jailed bhyve instance) can
use this vulnerability to achieve kernel code execution.

IV.  Workaround

No workaround is available.  This issue is likely of concern only to systems
relying on running bhyve in jail(8) for security domain separation.

V.   Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date,
and reboot.

Perform one of the following:

1) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for a security update"

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-20:28/bhyve_vmcs.patch
# fetch https://security.FreeBSD.org/patches/SA-20:28/bhyve_vmcs.patch.asc
# gpg --verify bhyve_vmcs.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/12/                                                        r365777
releng/12.2/                                                      r365779
releng/12.1/                                                      r365779
stable/11/                                                        r365777
releng/11.4/                                                      r365779
releng/11.3/                                                      r365779
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-24718>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-20:28.bhyve_vmcs.asc>
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